Make services.nscd non-forking

Systemd prefers non-forking services.

Another option is to use the -d flag. That also prevents forking and has
the side effect of making nscd log stuff to the journal. Unfortunately,
the nscd log is quite verbose (IMHO) and it seems to ignore the
debug-level option in nscd.conf, so I went with the -F (--foreground)
flag.

Observation: With -F, nscd still writes the pid file, but it doesn't
care if it already exists (so we don't need to remove the file before
the service starts). With -d no pid file is created.